How Quantum Mechanics Lets Us See, Smell and Touch

How the science of the super small affects our everyday lives.

If Max Planck hadn’t ignored some bad advice, he would never have started a
revolution. The pivotal moment happened in 1878, when young Planck asked
one of his professors whether to continue pursuing a career in physics.
Herr Professor Philipp von Jolly told Planck to find another line of work.

All the important discoveries in physics had already been made, the
professor assured his young protégé. As Planck later recalled, von Jolly
told him, “[Physics] may yet keep going in one corner or another,
scrutinizing or putting in order a jot here and a tittle there, but the
system as a whole is secured, and theoretical physics is noticeably
approaching its completion.”

Putting one of those jots in order, it turned out, eventually won Planck a
Nobel Prize — and led to the birth of quantum mechanics. The troublesome
trifle concerned a very ordinary phenomenon: Why do objects glow the way
they do when heated? All materials, no matter what they’re made of, behave
the same way with increasing temperature: turning red, then yellow, then
white. Yet no physicist in the 19th century could explain this seemingly
simple process.

The problem came to be called the ultraviolet catastrophe, because the best
theorem of the day predicted that objects heated to very high temperatures
should spew infinite amounts of short-wavelength energy. Since we know a
strong current doesn’t turn light bulbs and toasters into energy-spewing
death rays, 19th century physics clearly wasn’t the last word.

Planck found an answer in 1900 with what amounted to a modern-day hack. He
proposed (guessed, really) that energy could only be absorbed or emitted in
discrete packets, or quanta. It was a radical departure from so-called
classical physics, which held that energy flowed in smooth, continuous
streams. At the time, Planck had no theoretical justification — but it
turned out to work anyway. His quanta effectively capped the amount of
energy that heated objects could release at any temperature. No more death
rays.

So began the quantum revolution. It would take decades of incandescent
theoretical work by Albert Einstein, Werner Heisenberg, Niels Bohr and
other titans to transform Planck’s inspiration into a full theory, but it
all started because no one understood what happened to things when they get
hot.

The resulting theory, quantum mechanics, deals with particles and blips of
energy in the realm of the ultra-small, divorced from our everyday
experience, and all but invisible to our clumsy mammalian sensory
apparatuses. Well, not completely invisible. Some quantum effects are
hiding in plain sight, blatantly and beautifully obvious, like the sun’s
rays and the twinkling of the stars — something else that couldn’t be fully
explained before the advent of quantum mechanics.

How much of the quantum world can we experience in our daily lives? And
what sort of information can our senses glean about the true nature of
reality? After all, as the origin of the theory itself makes clear, quantum
phenomena can lie just under our noses. In fact, they may be taking place
right inside our noses.

The Quantum Schnozz

What’s going on in your nose when you wake up and smell the coffee, or the
slice of bread browning in your non-lethal toaster? For such an
in-your-face sensory organ, the nose is poorly understood. No less a
luminary than Enrico Fermi, who built the world’s first nuclear reactor,
once remarked to a friend while frying onions that it would be nice to
understand how our sense of smell works.

So you’re lying in bed, and someone has thoughtfully brewed some freshly
ground Sumatran dark roast. Molecules from the elixir waft through the air.
Your inhalations draw some of these molecules into a cavity between your
eyes just above the roof of your mouth. The molecules stick to a layer of
mucus on the upper surface of the cavity, embedded with olfactory neurons.
Dangling from the brain like the tentacles of a jellyfish, olfactory
neurons are the only part of the central nervous system constantly exposed
to the outside world.

Jay Smith

What happens next isn’t quite clear. We know the molecules bind to some of
the 400 different receptors on the surface of the olfactory neurons; we
don’t know exactly how that contact creates our sense of smell. Why is
smell such a difficult sense to understand?

“In part, it’s the difficulty of setting up experiments to probe what’s
going on inside the olfactory receptors of the nose,” says Andrew
Horsfield, a materials scientist at Imperial College London.

The conventional explanation for how smell works seems straightforward: The
receptors accept very specific shapes of molecules. They’re like locks,
which can be opened only by the right keys. Each of the molecules escaping
from your cup of joe, according to this model, fits into a particular set
of receptors in your nose. The brain interprets the unique combination of
receptors activated by their bound molecules as the smell of coffee. In
other words, we smell the shapes of molecules.

But there’s a fundamental problem with the lock-and-key model: “You can
have molecules of wildly different shape and composition, which all give
you the same odor perception,” says Horsfield. It seems that something more
than shape must be involved, but what?

A controversial alternative to the lock-and-key model suggests our sense of
smell arises not just from the shape of molecules, but also from the manner
in which those molecules vibrate. All molecules constantly jiggle with
distinct tempos, based on their structure. Could our noses somehow detect
differences in those vibrational frequencies? Luca Turin, a biophysicist at
the Alexander Fleming Biomedical Sciences Research Center in Greece,
believes they can.

Turin, who also happens to be one of the world’s leading perfume experts,
was inspired by a vibrational theory of smell first proposed by chemist
Malcolm Dyson in 1938. After Turin first caught scent of Dyson’s idea in
the 1990s, he started looking for molecules that would allow him to test
the theory. He hit upon sulfur compounds, which have a unique odor and a
characteristic molecular vibration. Turin then needed to identify a
completely unrelated compound — one with a different molecular shape than
sulfur but possessing the same vibrational frequency — to see if it would
smell anything like sulfur. Eventually, he found one, a molecule containing
boron. And sure enough, it reeked of sulfur. “That’s when the penny
dropped,” he says. “I thought, ‘This cannot be a coincidence.’ ”

It may be that none of our perceptions match what’s really out there

Since that odoriferous eureka moment, Turin has been gathering experimental
evidence to support the idea, collaborating with Horsfield to work out the
theoretical details. Five years ago, when Turin and colleagues designed an
experiment in which some of the hydrogen molecules in a musk-scented
fragrance were replaced with deuterium — a variety of hydrogen containing
an extra neutron — they found that people could smell the difference. Since
hydrogen and deuterium have identical shapes but different vibrational
frequencies, the results again suggested that our noses could indeed detect
vibrations. Similar experiments with fruit flies complemented those
results.

Turin’s idea remains contentious — his experimental data have divided the
interdisciplinary community of olfactory researchers. But if he is right,
and we do smell vibrations in addition to shapes, how do our noses manage
the feat? Turin speculates that a quantum effect called tunneling might be
involved.

In quantum mechanics, electrons and all other particles possess a dual
nature; each is both a particle and a wave. This sometimes allows electrons
to spread out and travel, or tunnel, through materials in ways that would
be forbidden to particles under the rules of classical physics. The
molecular vibrations of a scent molecule might provide the right jump down
in energy that electrons need to tunnel from one part of an odor receptor
to another. The tunneling rate would change with different molecules,
triggering nerve impulses that create the perceptions of different smells
in the brain.

Tucked away in our noses, then, might be a sophisticated electronic
detector. How could our noses have evolved to take advantage of such
quantum strangeness? “I think we underestimate the technology, so to speak,
of life by a couple of orders of magnitude,” says Turin. “Four billion
years of R&D with unlimited funding is a long time. And I don’t think
this is the most amazing thing that life does.”

Sight Unseen

OK, so you’re quaffing your coffee, nearly awake. Your eyelids are gearing
up for daytime mode, blinking, letting in a bit of the light that’s
streaming through the window. As you sip your brew, ponder this: The
particles of light warming your face and entering your eyes originated a
million years ago in the center of the sun, around the time our
not-quite-human ancestors started to use fire. The sun wouldn’t even be
sending out those particles, named photons, if not for the same phenomenon
that might underlie our sense of smell — quantum tunneling.

Some 93 million miles separate the sun and Earth, and it takes photons just
over eight minutes to cover that distance. But the bulk of their journey
occurs inside the sun, where a typical photon spends a million years trying
to escape. Matter is so tightly packed at the center of our star — the
hydrogen there is about 13 times denser than lead — that photons can travel
only an infinitesimal fraction of a second before being absorbed by a
hydrogen ion, which then spits the photon out for another
soon-to-be-interrupted journey, ad infinitum. After about a billion
trillion such interactions, a photon finally emerges from the surface of
the sun, having zigged and zagged randomly for a thousand millennia.

Jay Smith

But the photons never would have been born, and the sun wouldn’t shine,
were it not for quantum tunneling. The sun and all other stars generate
light by nuclear fusion, smashing hydrogen ions together to form helium, a
process that releases energy. Every second, the sun converts about 4
million tons of matter into energy. But hydrogen ions, single protons, have
positive electric charges and naturally repel each other. So how can they
possibly fuse?

With quantum tunneling, the wave nature of protons allows them to overlap
ever so slightly, like ripples merging on the surface of a pond. That
overlap brings the proton waves close enough so that another force — the
strong nuclear force, which kicks in only at extremely small distances —
can overcome the particles’ electrical repulsion. The protons fuse and
release a single photon.

Our eyes have evolved to be exquisitely sensitive to these photons. Some
recent experiments have shown that we can even detect single photons, which
raises an intriguing possibility: Could humans be used to test some of the
weird features of quantum mechanics? That is, could a person — like a
photon or an electron or Schrödinger’s hapless cat, dead and alive at the
same time — directly engage with the quantum world? What might such an
experience be like?

“We don’t know because no one has tried it,” says Rebecca Holmes, a
physicist at Los Alamos National Laboratory in New Mexico. Three years ago,
when she was a graduate student at the University of Illinois at
Urbana-Champaign, Holmes was part of a team led by Paul Kwiat that showed
people could detect short bursts of light consisting of just three photons.
In 2016, a competing group of researchers, led by physicist Alipasha Vaziri
at Rockefeller University in New York, found that humans can indeed see
single photons. Seeing, though, might not accurately describe the
experience. Vaziri, who tried out the photon-glimpsing himself, told the
journal Nature, “It’s not like seeing light. It’s almost a feeling, at the
threshold of imagination.”

Vic29/Shutterstock

In the near future, Holmes and Vaziri expect experiments that will test
what people perceive when photons are put into strange quantum states. For
example, physicists can coax a single photon into what they call a
superposition, where it exists in two different places simultaneously.
Holmes and her colleagues have proposed an experiment involving two
scenarios to test whether people might directly perceive a superposition of
photons. In the first, ho-hum scenario, a single photon would go into
either the left or right side of a person’s retina, and the person would
note on which side of the retina they sensed the photon. But in the other
scenario, a photon would be placed in a quantum superposition that would
allow it to do the seemingly impossible: travel to both the right and left
sides of the retina simultaneously.

Would the person then sense light on both sides of the retina? Or would the
interaction of the photon with the eye cause the superposition to
“collapse,” as physicists say, into one position or the other — and if so,
would it happen equally on the right and left side, as theory suggests?

“Based on standard quantum mechanics, the photon in the superposition
probably wouldn’t look any different to them than actually randomly sending
a photon to the left or the right,” says Holmes. If it turns out that
someone participating in the experiment did indeed perceive the photon in
both places simultaneously, would that mean the person herself was in a
quantum state? “You could say the observer was in a quantum superposition
for some vanishingly small amount of time,” says Holmes. “But no one has
tried this, so truly, we don’t know. That’s reason enough to do the
experiment.”

Feeling Your Way

Now back to that cup of coffee. The cup feels substantial, a solid chunk of
matter firmly in contact with the skin of your hand. But that’s an
illusion: We never really touch anything, at least not in the sense of two
solid slabs of matter coming together. More than 99.9999999999 percent of
an atom consists of empty space, with nearly all its stuff concentrated in
the nucleus.

When you exert pressure against the cup with your hand, the seeming
solidity comes from the resistance of electrons in the cup. Electrons
themselves don’t have any volume at all — they’re just fleeting,
zero-dimensional flecks of negative electric charge that surround atoms and
molecules like clouds. And the laws of quantum mechanics limit them to
specific energy levels around atoms and molecules. As your hand grasps the
cup, it forces electrons from one level to another, and that requires
energy from the hand’s muscles, which the brain interprets as touching
something solid.

Jay Smith

Our sense of touch, then, arises from an exceedingly complex interaction
between electrons around the molecules of our bodies and those of the
objects we encounter. From that information, our brain creates the illusion
that we possess solid bodies moving through a world filled with other solid
objects. Touch doesn’t give us an accurate sense of reality. And it may be
that none of our perceptions match what’s really out there. Donald Hoffman,
a cognitive neuroscientist at the University of California, Irvine,
believes that our senses and brain evolved to hide the true nature of
reality, not to reveal it.

“My idea is that reality, whatever it is, is too complicated and would take
us too much time and energy [to process],” he says.

Hoffman likens the picture our brain constructs of the world to the
graphical interface on a computer screen. All the colorful icons on the
screen — the trash can, the mouse pointer, the file folders — bear no
resemblance at all to what’s really going on inside the computer. They’re
abstractions, simplifications that allow us to interact with complex
electronics.

In Hoffman’s view, evolution has shaped our brains to operate in much the
same way, as a graphical interface that doesn’t reproduce the world with
any sort of fidelity. Evolution doesn’t favor the development of accurate
perceptions; it rewards ones that enhance survival. Or as Hoffman puts it,
“Fitness beats truth.”

Hoffman and his graduate students have run hundreds of thousands of
computer models in recent years to test his ideas. In the simulations,
artificial life-forms compete for limited resources. And in every case, the
organisms programmed to emphasize fitness outcompete the various ones
primed for accurate perceptions. For example, if one organism is tuned to
accurately perceive, say, the total amount of water present in an
environment, it will lose out to an organism that’s tuned to perceive
something simpler: the optimal amount of water needed to stay alive.

So while one organism might construct a more accurate representation of
reality, that representation doesn’t enhance its survivability. Hoffman’s
studies have led him to a remarkable conclusion: “To the extent that we’re
tuned to fitness, we will not be tuned to reality. You can’t do both.”

His ideas align with what some physicists believe to be a central message
of quantum theory: Reality is not completely objective — we cannot separate
ourselves from the world we observe. Hoffman fully embraces that view.
“Space is just a data structure,” he says, “and physical objects are
themselves also data structures that we create on the fly. When I look at
that hill over there, I create that data structure. Then I look away and
I’ve trashed that data structure because I don’t need it anymore.”

As Hoffman’s work shows, we haven’t yet come to grips with the full meaning
of quantum theory and what it says about the nature of reality. Planck
himself struggled for most of his life to understand the theory he helped
launch, and always believed in an objective universe that exists
independently of us. He once wrote about why he decided to go into physics
against the advice of his mentor: “The outside world is something
independent from man, something absolute, and the quest for the laws which
apply to this absolute appeared to me as the most sublime scientific
pursuit in life.” Maybe it will take another century, and another
revolution, to prove whether he was right, or as mistaken as Professor von
Jolly.